Optimal resetting of the transformer&#39;s core in single ended forward converters

ABSTRACT

The transformer&#39;s core in single ended forward converters is reset by a &#34;magnetizing current mirror&#34; consisting of a capacitor in series with an auxiliary switch which, during the OFF period of the primary switch, couples the capacitor to one of the transformer&#39;s windings to form a resonant circuit with the transformer&#39;s magnetizing inductance. The resonant circuit recycles the transformer&#39;s magnetizing energy by creating a mirror image of the magnetic flux between ON periods. This maximizes the flux swing available within a given core. The voltage stress on the primary switch is minimized as the voltage across the switch during the OFF period is approximately constant and automatically tailored to avoid dead time for arbitrary values of the switch duty cycle.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to DC-to-DC converters which process electricalpower from a source, at an input DC voltage, to deliver it to a load, atan output DC voltage, by selectively connecting a power transformer tothe source and the load via solid state switches. In particular, theinvention relates to converters of the forward type, in which the powertransformer is simultaneously connected to the source and the load. Moreparticularly, the invention relates to forward converters of the singleended type, in which the power flow from source to load is controlled bya single solid state switch.

2. Description of the Prior Art

This invention relates to the class of DC-to-DC converters whichincorporate the topology represented in FIG. 1. A converter in thatclass is referred to as a "single ended forward" coverter because powerflow is gated by a single switch 10 and energy is transferred forward,from the primary winding to the secondary winding of the transformer 11,during the ON period of the switch 10.

Converters in this class present a unique problem, in that theconversion topology does not inherently define the mechanism by whichthe transformer's core is to be reset during the OFF period of theswitch. The solution to this problem is not unique, as evidenced by themultiplicity of proposals found in the literature which, in order toimplement the reset function, complement the topology represented inFIG. 1 by differing choices of auxiliary circuitry. The differences areimportant since they affect the cost of the converter, as well as itsefficiency and power density.

The traditional approach, represented in FIG. 2a, has been to reset thecore via an auxiliary transformer winding connected with invertedpolarity in series with rectifier 13. The operation of this resetmechanism is illustrated in FIG. 2b, where, in addition to idealizedcomponent behavior, a one-to-one turn ratio between auxiliary andprimary windings has been assumed. This figure exemplifies a sequence oftwo ON periods, separated by an OFF period to enable the core to resetitself. The figure displays, as functions of time, the state of theswitch 10, the voltage V across the switch, and the current I throughthe auxiliary winding.

The first ON period is given by the time interval between t₁ and t₂.During this interval, the voltage V across the switch 10 vanishes andthe source voltage V_(o) is impressed upon the primary winding. Themagnetizing inductance controls the slope of the magnetizing current,flowing in the primary winding, and of the magnetizing energy whichaccumulates in the transformer's core. The current I vanishes, as therectifier 13 is reverse biased, in a blocking state, and thus keeps theauxiliary winding inoperative.

At time t₂, the opening of the switch 10 interrupts current flow in theprimary winding. Neglecting the effects of leakage inductance betweenprimary and auxiliary windings, the voltage V is clamped to 2 V_(o) asthe rectifier 13 becomes forward biased and begins to conduct themagnetizing current. The current I through the auxiliary winding is thenequal to the peak value I_(p) of the magnetizing current. Following timet₂, I decays as magnetizing energy is returned to the voltage sourceV_(o). At time t₃, the recycling of the magnetizing energy is completed,the current I vanishes, and, neglecting hysteresis, the magnetic fluxthrough the transformer's core is reset to zero. The time intervalbetween t₂ and t₃ is the core reset period. Having assumed a one-to-oneprimary to auxiliary turn ratio, this period equals the ON period t₂-t₁.

The remainder of the cycle, between times t₃ and t₄, is the "dead"period. In this period, the switch 10 remains open, the voltage V acrossit equals the source voltage V_(o), and the current I vanishes. Thecircuit in FIG. 2a is not efficiently functional during dead time.

The relative duration of the dead period depends upon the duty cycle ofthe switch 10 (assumed to be 33% in FIG. 2b). At 50% duty cycle, thedead period vanishes. Operation beyond 50% duty cycle would lead tosaturation of the transformer's core and (catastrophic) converterfailure.

Thus, the traditional reset mechanism, represented in FIG. 2a, presentsan inherent limitation in the available duty cycle range. This is asignificant drawback as it impairs the ability of the converter toregulate against wide variations in the source voltage or in the load.Another drawback of the traditional reset method is that allowed valuesof the duty cycle are in general associated with a non-vanishing deadtime. The existence of a dead time causes the switch 10 to experience apeak voltage greater than is in principle necessary to reset the core inthe time interval between ON periods.

Similar limitations apply, to a varying degree, to any other resetmechanism which involves a variable dead time to accomodate variationsin the switch duty cycle. Reset methods falling into this category arefound in S. Hayes, Proceedings of Powercon 8, Power Concepts Inc. 1981and in R. Severns, ibid..

To avoid these limitations, a different approach to the resetting of thetransformer's core in single ended forward converters was proposed by S.Clemente, B. Pelly and R. Ruttonsha in "A Universal 100 KHz Power SupplyUsing a Single HEXFET", International Rectifier Corporation ApplicationsNote AN-939, December 1980. These authors suggest acapacitor-resistor-diode network, as represented in FIG. 3a. The networkclamps the switch to the minimal peak voltage consistent with a givensource voltage and switch duty cycle, eliminating the need for dead timewhile allowing for a wide range of duty cycles. Attainment of thesedesign goals is actually dependent upon component characteristics andvalues. In particular, the resistor 15 must be sized small enough sothat the transformer's magnetizing current does not ever vanish.

With this assumption, the operation of this reset circuit is illustratedin FIG. 3b. As in the example given to illustrate the traditional resetmechanism, a sequence of two ON periods separated by an OFF period, witha 33% duty cycle is considered. The figure displays, as functions oftime, idealized waveforms defining the state of the switch 10, thevoltage V across it, and the current I through the rectifier 13. Duringthe OFF period, the latter coincides with the transformer's magnetizingcurrent.

As exhibited in FIG. 3b, the voltage V across the switch 10 is now arectangular waveform with a peak value equal to 1.5 V_(o). The current Ithrough the rectifier 13 is a trapezoidal waveform which, during the OFFperiod, decays from a peak value (I_(p) +I_(o)) to a minimum valueI_(o), a non-negative function of V_(o) and the duty cycle D.

A comparison of the voltage waveform of FIG. 3b with the correspondingone in FIG. 2b emphasizes the main advantage of thecapacitor-resistor-diode clamp: a reduction in the voltage stressapplied to the switch 10. A related advantage is the elimination ofbounds resulting from core saturation on the duty cycle range, enablingthe converter to remain functional over wider ranges of input voltageand output load. A further advantage is the avoidance of auxiliarytransformer windings which simplifies transformer construction.Unfortunately, while attaining these benefits, the reset mechanism ofFIG. 3a compromises the converter's efficiency and power density.

The reduction in the efficiency of the conversion process arisesprincipally from the dissipation of magnetizing energy accumulated inthe transformer during the ON period. Instead of being recycled, thisenergy is converted into heat by the clamp circuit. This power waste issignificant, particularly in an otherwise efficiency mindful conversionsystem.

The reduction in power density results mainly from an increase in thesize of the transformer which is rendered necessary by a decrease in theavailable dynamic flux swing for the magnetic material making up thetransformer's core. This is evidenced in FIG. 3b by the quantityreferred to as I_(o), which represents a non-negative, static componentof the magnetization current. The component shifts the peak value of themagnetizing current, leading to an excitation of the transformer's corewhich brings the magnetic material closer to saturation. The consequentdecrease in available flux swing reduces the power handling capabilityper unit volume at a given frequency of a given core and, with it, theconverter's power density.

From these points of view, the traditional reset mechanism of FIG. 2aoffers important, relative advantages. However, even from the point ofview of core utilization, it does not represent an optimal resetmechanism, since the flux swing is still unipolar. This unipolarcharacter of the transformer's core excitation has often been noted tobe an inherent drawback of single ended forward converters. In fact, itis not a general drawback of this class of conversion topologies as itis only inherent to some reset mechanisms which have been adopted tocomplement those topologies.

These considerations suggest that the "optimal" reset mechanism forsingle ended forward converters, yet to be invented, should incorporatethe following set of objectives:

it should be non-dissipative in nature, i.e. it should recycle thecore's magnetization energy;

it should maximize the available flux swing, i.e. it should lead to abipolar core excitation;

it should minimize the voltage stress on the switch, i.e. the voltagewaveform should be rectangular without involving a dead period;

it should not introduce constraints on the switch duty cycle;

it should simplify transformer construction by eliminating the need forauxiliary windings.

SUMMARY OF THE INVENTION

This invention provides new apparatus for resetting the transformer'score in single ended forward converters. The apparatus consists of astorage capacitor, an auxiliary solid state switch (distinguished fromthe primary switch which controls the converter's power flow), and of aswitch control circuit. The switch control circuit operates theauxiliary switch in its open state during the converter's ON period,when the primary switch is closed, and in its closed state during theconverter's OFF period when the primary switch is open. The auxiliaryswitch (operated by such a control circuit) and storage capacitor areconnected in parallel with a transformer winding.

The apparatus defined above resets the transformer's core byimplementing the conceptual function of a "magnetizing current mirror":it takes the magnetization at the end of the ON period and creates amirror image of it prior to the initiation of the following conversioncycle. The image is created via the charging and discharging of thestorage capacitor which forms a resonant circuit with the transformer'smagnetizing inductance. The capacitor is sized so that the period ofthis resonant circuit is considerably greater than the conversionperiod. Consequently, the capacitor's voltage and the voltage across theprimary switch are approximately constant during the OFF period.

The new apparatus provides optimal resetting of the transformer's corein single ended forward converter topologies:

it is non-dissipative, as it recycles the core's magnetization energyvia intermediate storage in a resonant circuit;

it maximizes the available flux swing, as it creates a mirror image ofthe magnetic flux between ON periods;

it minimizes the voltage stress on the (primary) switch, as it appliesto this switch during the OFF period an approximately constant voltagewhich is automatically tailored to avoid dead time;

it does not introduce constraints on the switch duty cycle due to coresaturation;

it simplifies transformer construction, as it can be implemented withoutauxiliary windings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 defines the general class of single ended forward converters.

FIG. 2a shows the auxiliary transformer winding which has beentraditionally employed to reset the core in single ended forwardconverters.

FIG. 2b exemplifies the operation of the reset mechanism in FIG. 2a bydisplaying a possible time sequence of states for the switch 10 and thecorrresponding idealized waveforms for the voltage V, across the switch10, and the current I, in the auxiliary transformer winding.

FIG. 3a shows a capacitor-resistor-diode network which has been employedin the prior art as an alternative mechanism for resetting the core insingle ended forward converters.

FIG. 3b provides an example of the operation of the reset mechanism inFIG. 3a analagous to that of FIG. 2b to allow for a direct comparison ofvoltage and current waveforms.

FIG. 4a discloses a preferred embodiment of the new reset mechanism forsingle ended forward converters, consisting of a "magnetizing currentmirror" connected across the transformer's secondary.

FIG. 4b discloses voltage and current waveforms useful in describing theoperation of the new reset mechanism.

FIG. 4c discloses a preferred implementation of the magnetizing currentmirror in which the auxiliary switch is realized in terms of a MOSFETtransistor and its integral reverse rectifier.

FIG. 4d discloses equivalent circuit diagrams characterizing the ON andOFF periods of a single ended forward converter reset by a magnetizingcurrent mirror.

FIG. 4e discloses an embodiment of the new reset mechanism in which themagnetizing current mirror is connected across the transformer'sprimary.

FIG. 4f discloses yet another embodiment of the invention in which themagnetizing current mirror is connected across an auxiliary transformerwinding.

FIG. 5 compares idealized waveforms exemplifying the time evolution ofthe magnetic flux across the transformer's core in a single endedforward converter reset by: (a) the traditional mechanism, employing anauxiliary transformer winding; (b) the capacitor-resistor-diode network;(c) the optimal reset mechanism, utilizing a magnetizing current mirror.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 4a, the primary switch 10 selectively couples theprimary winding of transformer 11 to a source of voltage V_(o). Arectifier 12 is connected in series with the secondary winding oftransformer 11 and is oriented to conduct a current during the ON periodof the primary switch 10. These are conventional elements of a singleended forward converter. In order to reset the transformer 11 during theOFF period of the primary switch 10, these elements are complemented bya "magnetizing current mirror."

The magnetizing current mirror comprises the storage capacitor 20, theauxiliary switch 21 and the switch control circuit 22. The capacitor 20and the switch 21 are connected in series. The switch 21 is operated bythe control circuit 22 in accordance with a control logic requiring thatthe auxiliary switch 21 be opened prior to the ON period of the primaryswitch 10, and closed after this period. To accomplish this function, assuggested by the arrows at the bottom of the box representing the switchcontrol circuit 22, this circuit is interfaced with primary switchcontrol circuitry, not represented in the figure. The implementation ofthe control circuit and of its interface can be realized in a number ofways which will become obvious to those skilled in the art.

In FIG. 4a, the magnetizing current mirror is connected in parallel withthe secondary winding of transformer 11. Assuming an equal number ofturns between this and the primary winding, and neglecting the effectsof leakage inductance between primary and secondary windings, andparasitic effects including the ones associated with winding capacitanceor the capacitance of non-ideal hardware realizations of the primaryswitch 10 and auxiliary switch 21, the operation of the magnetizingcurrent mirror as a reset mechanism is illustrated by an example in FIG.4b.

As in the examples of FIG. 1b and 2b which were used to characterize theoperation of reset mechanisms found in the prior art, FIG. 4b considersa sequence of two ON periods separated by an OFF period, with a 33% dutycycle. The figure displays, as functions of time, idealized waveformsdefining the state of the switch 10, the voltage V across it and thecurrent I through the auxiliary switch 21.

At time t₁, the auxiliary switch 21 is opened and the primary switch 10is closed, initiating the first ON period. During this period, thevoltage V across the primary switch and the current I through theauxiliary switch vanish. The source voltage V_(o) is impressed upon theprimary winding of transformer 11, causing the magnetic flux φ acrossthe transformer's core to change with time as dictated by Faraday's law.If N is the number of primary (and secondary) turns, the total change inthe flux φ is V_(o) (t₂ -t₁)/N. Concurrent with this is a change in themagnetizing current given, in terms of the magnetizing inductance L_(M),by V_(o) (t₂ -t₁)/L_(M).

At time t₂, the primary switch 10 is opened and the auxiliary switch 21is closed, initiating the OFF period. During this period, the voltageacross the auxiliary switch and the current through the primary switchvanish. The voltage V across the primary switch is clamped to a valueV_(p) =V_(o) +V_(c), where V_(c) is the voltage across the storagecapacitor 20. Conduction of the magnetizing current is transferred tothe secondary winding where the current loop is closed by the storagecapacitor 20 and the auxiliary switch 21. Initially, this current, I, isnegative in sign and equal in magnitude to I_(p) /2.

The evolution of the system during the OFF period, the time intervalbetween t₂ and t₄, depends upon the capacitance value C chosen for thestorage capacitor 20. C should be chosen to be large enough so that thetime dependence of the voltage, V_(c), across the capacitor can beapproximately neglected. This accounts for the constancy of the voltageV, and the linear rise of the current I, both displayed in FIG. 4b.Invoking once again Faraday's law, the total change in the magnetic fluxφ during the OFF period is then approximately given by -V_(c) (t₄-t₂)/N. By equaling the magnitude of this change to the correspondingflux change during the ON period, it follows that

    V.sub.p ≃V.sub.o (1+D)                       (1)

where D=(t₂ -t₁)/(t₄ -t₂) is the primary switch duty cycle. The totalchange in the magnetizing current I during the OFF period isapproximately given by -V_(c) (t₄ -t₂)/L_(M). Since the integral of themagnetizing current I during the OFF period must vanish (under steadystate conditions), it follows that

    I.sub.p /2≃V.sub.o t.sub.ON /(2L.sub.M),     (2)

where t_(ON) =(t₂ -t₁) is the ON time of the primary switch.

The evolution of the system during the OFF period may be analyzedfurther by dividing the period into two intervals, t₂ -t₃ and t₃ -t₄, ofequal duration, characterized respectively by negative and positivevalues of the magnetizing current I. In the first interval, themagnetizing current charges the storage capacitor 20, and storage ofmagnetizing energy is progressively transferred from the transformer tothe capacitor. This process is completed at time t₃ when the magnetizingcurrent vanishes. In the second interval, the magnetizing currentdischarges the storage capacitor 20, and storage of magnetizing energyis progressively transferred back from the capacitor to the transformer.This process is completed at time t₄ when a mirror image of themagnetizing current has been formed and magnetizing energy has beenreflected into the transformer, resetting it for the next cycle.

Because of the alternating character of the magnetizing current I, theauxiliary switch 20 must be able to conduct negative as well as positivecurrents, in addition to being able to block positive voltages. Thisobservation suggests MOSFET transistors as natural candidates toimplement the functions of the switch 21.

FIG. 4c shows a magnetizing current mirror in which the auxiliary switch21 is implemented with a MOSFET transistor. The reverse rectifierinherent to the MOSFET is explicitly shown. Thus the auxiliary switch 21can be thought of as being always closed to negative currents andselectively closed to positive currents. The flow of positive currentsis controlled by the switch control circuit 22 which applies a suitablevoltage to the gate of the MOSFET.

Referring back to the example of FIG. 4b, it is apparent that the MOSFET21 can be turned on at any time between t₂ and t₃ without disrupting theoperaion of the magnetizing current mirror. Such a delay does notrepresent "dead" time since during this time the reset mechanism isoperational. On the other hand, a delay between the opening of theauxiliary switch 21 and the closing of the primary switch 10 representsdead time. For this reason it is efficient to keep such a delay to aminimum, consistent with the requirement to avoid an overlap betweenswitches. However, a small delay is useful to allow the magnetizingcurrent to charge and discharge parasitic capacitances associated withthe switches and windings.

A different perspective on the operation of the magnetizing currentmirror as a reset mechanism for single ended forward converters isoffered by FIG. 4d showing equivalent circuit diagrams characterizingthe converter's ON and OFF periods. The magnetizing current mirror isoperational only during the OFF period, when the storage capacitor 20 isconnected via the auxiliary switch 21, in its closed position, across awinding of transformer 11. These elements form a resonant circuit ofperiod T_(res) =2π·√L_(M) C, where L_(M) is the transformer'smagnetizing inductance as seen from the mirror and C is the mirror'scapacitance. During the OFF period of the converter's cycle, theresonant circuit undergoes a portion of its natural cycle which, understeady state conditions, is centered about a voltage maximum.Approximate constancy of the voltage seen by the primary switch 10during the OFF period translates into the requirement that the OFFperiod t_(OFF) be small relative to the resonant period T_(res), t_(OFF)<<T_(res).

The condition t_(OFF) <<T_(res) should, however, not be interpreted tomean that the mirror's capacitance should be made arbitrarily large,since the resonant period T_(res) introduces a time scale which limitsthe converter's transient response time. As implied by Eq. (1), a changein the converter's duty cycle leads to a change in the voltage acrossthe storage capacitor 20. To effect the latter, the integral of themagnetizing current during the OFF period is non-vanishing.Consequently, under transient conditions Eq.(2) ceases to be applicable.To avoid transformer saturation the peak value of the magnetizingcurrent must then be limited by limiting the rate of change of theconverter's duty cycle and, therefore, the transient response time.

Aside from transient conditions, during which the voltage V_(c) acrossthe storage capacitor 20 changes to adjust itself to a varying dutycycle, the magnetizing current mirror and the transformer's core definean essentially closed system: magnetizing energy transferred from thetransformer to the storage capacitor is injected back into thetransformer within the converter's OFF period. This internal recyclingis only incomplete to the extent that non-ideal circuit elements giverise to energy losses. These may be accounted for by modifying theresonant circuit of FIG. 4d with the addition of resistive componentsrepresenting the effects of losses associated with the transformer'score, the winding resistance and the equivalent series resistances ofthe storage capacitor 20 and the auxiliary switch 21.

The equivalent circuits of FIG. 4d suggest that applications of themagnetizing current mirror to the resetting of the transformer's core insingle ended forward converters need not be restricted to the topologyof FIG. 4a. Useful variations of the new reset mechanism are indeedobtained by connecting the mirror in parallel with different transformerwindings.

In FIG. 4e, the magnetizing current mirror is connected in parallel withthe primary winding of transformer 11. The main advantage of thistopology, relative to that of FIG. 4a, originates from a direct couplingof the mirror to the primary switch 10. This eliminates a certain amountof ringing of the voltage V across the primary switch, due to leakageinductance and parasitic capacitances, which is present with thetopology of FIG. 4a. However, this is not a serious problem. On theother hand, a MOSFET implementation of the switch 21 in FIG. 4e wouldrequire the use of a p-channel device and/or a floating gate drive. Thisis a relatively serious drawback for this topology.

In FIG. 4f, the magnetizing current mirror is connected in parallel withan auxiliary transformer winding. Possible advantages to thisconfiguration originate from the flexibility provided by the choice ofturn ratio between auxiliary and primary windings, and the possibilityto magnetically couple these windings closely. The trade off is addedcomplexity in transformer construction.

These and other possible variations in the detailed implementation ofthe new reset mechanism share the same equivalent circuits of FIG. 4dand the same fundamental advantages when compared to reset mechanismsknown in the prior art. Some of these advantages are made more evidentby referring to FIG. 5, which compares the idealized behavior of themagnetic flux as it would evolve in the examples of FIGS. 2b, 3b and 4b,corresponding to: (a) the traditional reset mechanism; (b) thecapacitor-resistor-diode network; (c) the new reset mechanism. Thecurves denoted respectively by a, b and c define as a function of timethe flux across a core made of soft ferromagnetic material of negligiblehysteresis. The saturation flux is denoted by φ_(sat).

Among the three reset mechanisms considered in FIG. 5, thecapacitor-resistor-diode network (b) is the one that brings the coreclosest to saturation and does not recycle the core's magnetizationenergy. This qualifies this reset mechanism as the most inefficient inutilizing space (the volume of the core) and energy. Its redeemingfeature is the constancy in the slope of the flux curve between t₂ andt₄ which, in view of Faraday's law, implies minimal voltage stress onthe converter's primary switch.

Some of the drawbacks of the traditional reset mechanism (a) stem fromthe greater slope of the flux curve between t₂ and t₃ and vanishingslope between t₃ and t₄. These observations imply greater voltage stresson the primary switch, the presence of dead time, and a (50%) limitationon the duty cycle. Other drawbacks stem from the asymmetry betweenpositive and negative flux, which signifies inefficient use of thecore's volume and increased core energy losses.

In light of these considerations, the nature of curve c, characterizingthe new reset mechanism, speaks for itself and suggests that amagnetizing current mirror should provide optimal resetting of thetransformer's core in single ended forward converters.

Other embodiments are within the following claims.

I claim:
 1. In a single ended forward converter in which energy istransferred from a primary winding to a secondary winding of atransformer during the ON period of a primary switch, circuitry forrecycling the magnetizing energy stored in said transformer to reset itduring the OFF period of said primary switch, comprising:a storagecapacitor; an auxiliary switch connected in series with said storagecapacitor; a switch control circuit operating said auxiliary switch inaccordance with a control logic such that (a) said auxiliary switch isopened prior the ON period of said primary switch, (b) said auxiliaryswitch remains open throughout the ON period of said primary switch, (c)said auxiliary switch is closed after the ON period of said primaryswitch.
 2. The transformer resetting apparatus of claim 1 wherein saidcircuitry is connected in parallel with said secondary winding.
 3. Thetransformer resetting apparatus of claim 1 wherein said circuitry isconnected in parallel with said primary winding.
 4. The transformerresetting apparatus of claim 1 wherein said transformer further includesan auxiliary winding, wherein said circuitry is connected in parallelwith said auxiliary winding.
 5. The transformer resetting apparatus ofclaim 1 wherein said auxiliary switch is a MOSFET transistor with anintegral reverse diode.
 6. In a single ended forward converter in whichenergy is transferred across a transformer during the ON period of aprimary switch, an apparatus for recycling the magnetizing energy ofsaid transformer during the OFF period of said primary switch,comprising:a storage capacitor; auxiliary switching means to selectivelycouple said storage capacitor to said transformer, wherein said storagecapacitor and said transformer form a resonant circuit during the OFFperiod of said primary switch.